Photodynamic therapy (PDT) is a cancer treatment that utilizes visible or near-infrared light to activate a light- sensitive drug (photosensitizer, PS) that, in turn, creates reactive singlet oxygen species from ground triplet state oxygen that is present in the tumor. The resulting reactive products induce tissue death. Ideally the reactive species will be produced within only the target volume, leading to damage of the tumor or diseased tissues, while minimizing damage to surrounding normal tissue. Unlike chemotherapy, PDT does not cause systemic toxicities, and unlike radiation therapy it does not cause cumulative damage in the local field. When an external light beam is used to treat superficial surface or intracavitary surface cancers, the effective depth of light penetration and treatment is limited to < 10 mm. For deeply seated tumors or tumors that are more than 10 mm in thickness, intra-tumor light delivery (interstitial PDT, I-PDT) is required to activate the PS.